Conventionally, there have been measured circuit parameters (such as the S parameters) of a device under test (DUT) (refer to Patent Document 1: Japanese Laid-Open Patent Publication No. H11-38054, for example).
Specifically, a signal is transmitted from a signal source to a receiving unit via the DUT. The signal is received by the receiving unit. It is possible to acquire the S parameters and frequency characteristics of the DUT by measuring the signal received by the receiving unit.
On this occasion, measurement system errors are generated in the measurement due to mismatching between a measurement system such as the signal source and the DUT, and the like. These measurement system errors include Ed: error caused by the direction of bridges, Er: error caused by frequency tracking, and Es: error caused by source matching.
On this occasion, it is possible to correct the errors according to Patent Document 1, for example. The correction in this way is referred to as calibration. A brief description will now be given of the calibration. Calibration kits are connected to the signal source to realize three types of states: open, short-circuit, and load (standard load Z0). In these states, a signal reflected from the respective calibration kits is acquired by a bridge to obtain three types of the S parameter corresponding to the three types of state. The three types of variable Ed, Er, and Es are acquired from the three types of the S parameter.
Moreover, a measurement-system error factor Et in the receiving unit can be acquired by directly connecting the signal source and the receiving unit, and measuring an S parameter of a signal received by the receiving unit (refer to Patent Document 2: WO 2003/087856 pamphlet, for example). It should be noted that a circuit element (a calibration kit) which realizes the direct-connection state is referred to as through.
The correction is carried out by employing the error factors Ed, Er, Es, and Et acquired in this way.
However, the error factors may not be correctly acquired due to a secular change in S parameters of calibration kits themselves (open, short-circuit, load, and through), and faulty characteristics of the calibration kits themselves (both are referred to as “fault of calibration kits”). It is thus necessary to check whether a fault has been generated in the calibration kits.
There is the following possible method for checking a fault in the calibration kits. First, the calibration kits are connected to the signal source, thereby obtaining the error factors Ed, Er, Es, and Et. Then, the signal from the signal source is transmitted to the receiving unit via a check path (S parameters of the check path are known). The signal is received by the receiving unit. The S parameters of the check path are acquired by measuring the signal received by the receiving unit, and correcting the signal based on error factors which have been acquired. The acquired S parameters of the check path and the known S parameters of the check path are compared with each other. As a result of the comparison, if both of them coincide with each other, it can be considered that the error factors can be correctly acquired, and a fault has not been generated in the calibration kits. On the other hand, as a result of the comparison, if both of them do not coincide with each other, it can be considered that the error factors cannot be correctly acquired, and a fault has been generated in the calibration kits.
However, the above-described method of checking a fault in the calibration kits requires the check path, which complicates the measuring system.
It is therefore an object of the present invention to easily check a fault in calibration kits in a measurement system.